An imaging object of an MRI device which becomes widespread in clinical application is protons which are a major constituent material of an inspection subject. Through imaging such as a spatial distribution of proton density and a spatial distribution of relaxation attenuation of excitation state, configurations or functions of such as a human head, abdomen and extremity are imaged in two dimension or three dimension.
Protons exist such as in water and fat in human tissue, however, their chemical shifts differ depending on their combination configurations. By making use of such chemical shift difference many approaches of drawing separately an image of protons in water and an image of protons in fat have been proposed. For example, as an example method of acquiring a fat suppressed image, a method, in which a plurality of images having different echo times (TE) are obtained and then water and fat separated images are acquired through computation thereof, is enumerated. A typical method therefor is disclosed in “Simple Proton Spectroscopic Imaging” by W. Thomas Dixon et al., (RADIOLOGY Vol. 153, pp 189–194 (1984)), which hereinbelow will be referred to as Dixon method. Methods of acquiring water and fat separated images through computation other than Dixon method are known and disclosed, for example, in the following papers “Water-Fat Imaging with Three-Point Direct Phase Encoding” by Qing-San Xiang and Li An, (Proc., SMR 3rd Meeting. p 658 (1995)), “Quadrature 2-point Water-Fat Imaging”, by Li An and Qing-San Xiang, (Proc., ISMRM 4th Scientific Meeting, p 1541 (1996)), and “Water-Fat Imaging with Three-Orthogonal-Phase Acquisitions” by Li An and Qing-San Xiang, (Proc., ISMRM Scientific Meeting, p 1866 (1998)).
These methods are common in the following aspect, in which image data of a plurality of images are acquired from a plurality of echo signals having different times (echo time TE) from nuclear spin excitation to generation of signals and the imaging is performed by separating water signals and fat signals through computation of the acquired image data.
These methods in which the water and fat separated images are acquired by performing computation with regard to the plural images prepared from the plural signals having such different echo times TE include the following problems. One problem is that an unintended phase offset is caused in the signal due to such as inhomogeneous static magnetic field and local magnetic field turbulence, and another problem is that an image obtained by the computation is difficult to discriminate between a water image and a fat image.
The first problem is caused by such as distortion of a magnet which generates the static magnetic field and the performance limitation of the magnet itself as well as may be caused by magnetic susceptibility difference in respective portions of an inspection subject, when the same being placed in an MRI device. A static magnetic field inhomogeneity in Field of View (FOV) of an MRI image causes to vary frequencies of MR signals and causes an image quality deterioration such as a position displacement and flow in the acquired image. Further, because of phase variation in images due to the static magnetic field inhomogeneity it is difficult of obtain a correct result, when performing a complex computation between images.
In connection with the above referred to water and fat image separation, as a method of resolving the problem with regard to the phase offset due to the static magnetic field inhomogeneity, such as 2-point Dixon method and 3-point Dixon method in which a function of correcting the influence due to the static magnetic field inhomogeneity is added to the Dixon method are proposed, for example, in “Two-Point Dixon Technique for Water-Fat Signal Decomposition with BO Inhomogeneity Correction” by Bernard D. Cooms et al., (Magnetic Resonance in Medicine, Vol. 38, pp 884–889 (1997)).
The above referred to method will be explained in connection with 2-point Dixon method. In the 2-point Dixon method, signals are acquired at timings when phases of proton in water and proton in fat are in in-phase and in anti-phase due to their chemical shift difference as illustrated in FIG. 1. In FIG. 1, 102 and 103 are respectively gradient magnetic field pulses for generating echo signals S1 and S2, and in the signal S1 a signal component 105 from water protons and a signal component 104 from fat protons are contained and in the signal S2 a signal component 107 from water protons and a signal component 106 from fat protons are contained.
Herein, the timing of acquiring the first echo signal (first echo) S1 is determined at a timing when 2nτ (wherein n is a positive integer, which is also true throughout the present specification) has elapsed after a high frequency magnetic field pulse 101 is generated, wherein when assuming difference of resonance frequencies of water protons and fat protons is Δf, 2τ=1/Δf, and the timing of acquiring the second echo signal (second echo) S2 is when τ has elapsed after the first echo.
When no phase offset due to the above referred to static magnetic field inhomogeneity is induced until the acquisition of the first and second echoes after generation of the high frequency magnetic field pulse 101, a water image and a fat image are obtained through computation between an image (first echo image) obtained from the first echo signal and an image (second echo image) obtained from the second echo signal according to the following equations:S1(x,y)=W(x,y)+F(x,y)  (1)S2(x,y)=W(x,y)−F(x,y)  (2)S1(x,y)+S2(x,y)=2W(x,y)  (3)S1(x,y)−S2(x,y)=2F(x,y)  (4)wherein, S1(x, y) represents the first echo, S2(x, y) represents the second echo, and W(x, y) and F(x, y) respectively represent magnitudes of signal due to water protons and of signal due to fat protons in the respective echo signals.
Now, when a phase offset exists in the signals, the first echo signal and the second echo signal are expressed as follows;S1(x,y)=(W(x,y)+F(x,y))exp(i(α(X,y)))  (5)S2(x,y)=(W(x,y)−F(x,y))exp(i(α(X,y)+φ(x,y)))  (6)wherein, α(x, y) is a phase rotation component due to such as inhomogeneity of RF magnetic field pulse in the vector direction, but independent from time and, in the case of the gradient echo (GrE) sequence as illustrated in FIG. 1, contains a phase rotation component caused during the time TE (wherein 2nπ) due to the static magnetic field inhomogeneity, and φ(x, y) is a phase rotation component due to the static magnetic field inhomogeneity.
As will be seen from the above, where there exists a static magnetic field inhomogeneity, a difference in phases of the first echo and the second echo is caused, thereby, the water signal and the fat signal can not be separated through the simple addition and subtraction as with the equations (3) and (4). Accordingly, in the Dixon method in which the function of correcting the influence due to the static magnetic field inhomogeneity is added, at first a phase offset φ(x, y) due to the static magnetic field inhomogeneity is determined through computation between two echoes, then, after correcting the phase offset, the water and fat image separation is performed through addition and subtraction.
The 2-point Dixon method with static magnetic field correction makes use of the fact that the phase difference between a water signal and a fat signal in S2 signal is π in order to determine the phase offset. Namely, when doubles π makes 2π, therefore, in view of principal value rotation the doubled value becomes equivalents to that with no rotation. Therefore, through subtracting the phase of S1(x, y) from that of S2(x, y) and doubling the resultant difference, a static magnetic field inhomogeneity map can be determined.
Further, in 3-point Dixon method three signals S1, S2 and S3 each having different echo time is obtained as shown in FIG. 2 and a phase rotation amount 2φ(x, y) is determined depending on a ratio between the first echo S1 and the third echo S3 in which the water signals and the fat signals are in in-phase. In FIG. 2, 202, 203 and 204 are respectively gradient magnetic field pulses for causing the echo signals S1, S2 and S3, and 206, 209 and 212 are water signals and 205, 208 and 211 are fat signals.
When determining the phase rotation amount due to the static magnetic field inhomogeneity in such Dixon methods with static magnetic field correction, a processing for eliminating the principal value rotation, namely a processing called as unwrapping or rewinding is required. Methods of unwrap processing are disclosed in the above referred to papers as well as the following papers; “Direct Calculation of Wrap-Free Phase Image” by M. Patel and X. Hu, (Proceedings of Annual Meetings of the Society of Magnetic Resonance in Medicine (=SMRM). No. 721, (1993)) and “Phase unwrapping in the Three Point Dixon Method for Fat Suppression MR Imaging” by Jerzy Szmowski et al., (Radiology, Vol. 192, pp 555–561 (1994)).
However, such unwrapping (rewinding) shows a problem of being susceptible to influence of noises. In particular, with regard to the 2-point Dixon method since the second echo signal has to be obtained at the timing when the phases of the water protons and the fat protons are in anti-phase, the difference between the water signal and the fat signal forms the second echo signal, therefore, of which intensity is small which is significantly affected by noises. In order to eliminate such influence of noises, some measures such as omitting the unwrap processing by masking regions which are susceptible to noises is necessitated, however, since such regions which are susceptible to noises vary depending on respective images, it was difficult to set and select proper noise removal masks for every unwrap processing.
With regard to the second problem that it is difficult to discriminate whether an image obtained by the computation is a water image or a fat image, such discrimination is theoretically possible, if the unwrap starting point is optimized in order to eliminate the phase offset of 2nπ caused in association with the unwrap processing. However, since there are no methods of optimizing automatically the unwrap starting point, measures of such as visually designating the starting point and of deciding which is the water image by visually observing the resultant image are taken and any automatic separation between a water image and a fat image is not realized until now.
Accordingly, an object of the present invention is to provide an MRI device provided with a function of acquiring a water image and a fat image through computation between images obtained from plural echo signals having different echo times which permits static magnetic field correction including a proper unwrap processing and further permits an automatic discrimination between the water image and the fat image. Thereby, another object of the present invention is to provide an MRI device which permits an automatic acquisition of separated water and fat images.
Further, still another object of the present invention is to provide an MRI method with imaging method including a static magnetic field correction processing in an MRI device which permits automatic optimization of unwrap processing.